1. Field of the Invention
The present invention relates to self-synchronous electrical motors that can be used notably in electrical traction systems for automobiles.
It will be recalled that a self-synchronous motor is a motor in which a magnetic field is produced at the stator, said magnetic field rotating at exactly the same speed as the rotor, whatever may be this speed, i.e. especially when the motor is started up. A sensor of the rotor position is used to synchronize the rotating magnetic field created at the stator.
In the field of the technology of electrical traction for automobiles, there is the major problem of reducing the quantity and cost price of the raw material used as well as the number of electronic components used for the controls.
The aim of the present invention is to achieve a self-synchronous electrical motor that can be used in an electrical traction system for automobiles, this motor being cheaper and more reliable than prior art motors while, at the same time, meeting standards laid down by regulations and especially safety standards.
2. Description of the Prior Art
Known self-synchronous motors comprise generally star-connected stator windings that are series-connected with electronic switches. These switches are actuated at a frequency (that can be called a high frequency) which is far higher than the electrical frequency of the rotating magnetic fields generated at the stator; the (low) frequency of the rotating magnetic fields corresponds to the speed of rotation of the motor. The switches are used to chop the current at high frequency with a cyclical ratio that is variable so that it is possible, firstly, to control the value of the mean current in the stator windings during periodic time intervals (low frequency) when a current effectively flows in these windings and, secondly, to totally interrupt this current during other periodic time intervals (low frequency). With several stator windings and, hence, several switches, it is possible to shunt or route the current successively (at low frequency) into the windings so as to create a rotating stator magnetic field. The envelope of variation of the mean current in a winding is defined by the variation of the cyclical ratio of the chopped current, which can be adjusted at will as a function of the waveform desired for the mean current. These self-synchronous motors use at least as many electronic switches as windings and these electronic switches are costly especially if high currents go through them, which is the case in automobiles.
The present invention is aimed at reducing the cost of the motor and increasing its reliability without lowering its performance characteristics.